The present invention is concerned with signal-seeking tuning systems for use in television receivers, and is more particularly concerned with a system for identifying a television signal and providing a control signal to a motor-tuned station selector in an intercarrier television receiver for selectively interrupting the operation of a signal-seeker at points of correct tuning in a predetermined television frequency spectrum.
Signal-seeking tuning systems for use in television receivers have been suggested heretofore. Such systems commonly employ a motor-driven signal seeker which is connected to the tuner of a television receiver, and which is energized to commence a continuous variation in the tuning of said receiver over a predetermined television frequency spectrum in response to momentary actuation of a start switch forming a portion of the system. The signal seeker continues its tuning operation until a point of correct frequency reception has been achieved, corresponding to the frequency of the transmitted television signal in the area of reception having a signal level sufficiently high to be properly reproduced by the receiver, at which time the receiver generates a control signal which functions to de-energize the motor-driven signal seeker to stop tuning of the receiver. The same type of general operation may be achieved, moreover, by using voltage-controlled tuners employing varactor diodes, with a progressively-variable voltage generator being used instead of a scanning motor. All such known systems constitute "signal seekers" within the meaning of the term as employed herein.
During a continuous tuning operation of the type described above, points of tuning will be achieved where a control signal, operative to stop the signal seeker, will be generated by image frequency heterodyning rather than by the reception of proper signal transmissions. The art has recognized this problem and efforts have been made to devise circuits which will distinguish between correct control signals resulting from the reception of a proper television transmission, and incorrect control signals which have resulted from image frequency heterodyning. The theories upon which these prior circuits have been based vary, with the result that the circuits have achieved image frequency rejection with varying degrees of reliability. As a general matter, however, prior art circuits have become increasingly reliable to achieve image frequency rejection only at the expense of increasingly complex and costly circuit configurations.
The present invention is intended to obviate these problems in the prior art by the provision of a signal identifier, for use in a signal-seeking tuning system, which is far simpler than signal identification circuits suggested heretofore, but which nevertheless achieves completely reliable operation and rejection of improper control signals resulting from image frequency heterodyning.